This invention relates to electronic surveillance systems, and more in particular to an improved marker for use in such systems.
Electronic surveillance systems of the type to which the present invention is directed, are generally employed to detect the presence of a magnetic marker in a magnetic field. Such systems thus include a device for generating a magnetic field, and a receiver for detecting variations in the field resulting from passing of a marker, generally carried by an article, through the field.
Magnetic markers for electronic surveillance systems are disclosed in detail, for example, in U.S. Pat. Nos. 4,510,489 and 4,510,490 Anderson et al, wherein the marker is comprised of at least one strip of a ferromagnetic material. In order to produce a resonant signal from the marker, the marker is of a magnetostrictive material. A magnetic bias is applied to the magetostrictive strip by positioning the strip in the magnetic field of at least one magnet of a high coercivity material.